If tractor head breaks down you can use another tractor to haul the trailer, greatly reducing down time. If self propelled breaks down the entire unit is out of service until vehicle is repaired.

For self propelled, the whole unit is sent to the service facilities until it is repaired or serviced and it exposes the body to unfavorable conditions at workshop.

Turning radius of a trailer is less than a self-propelled and more maneuverable especially in urban areas.

There is more square footage in a trailer for rooms/plumbing/ generators, etc..

A custom built trailer solution will last an average of 10 years longer due to the construction and durability

Finally, consider this important note:

This concept of using trailers for large fleets of mobile units is confirmed every day in the U.S. and Europe. Companies like Alliance Imaging in the U.S. and U.K. will only use fleets of trailers for their large number of mobile medical units (MRI, CT, PET/ CT, and other technologies). Their fleets number in the hundreds. In addition, there are many other similar fleet op- erators of mobile medical care delivery who operate only trailer fleets. They recognize the advantage of standardization of design, flexibility of using a variety of towing tractors preventing loss of revenue and service due to tractor breakdowns, maneuverability.